1. Field of the Invention
This invention relates to communication systems. In particular, the invention relates to robust frequency tracking in a communication system.
2. Description of Related Art
A receiver in a mobile communication system receives a signal transmitted from a base station over a known frequency band. Typically, the receiver has an acquisition and tracking loop circuit to lock on the transmitted signal. The acquisition and tracking loop circuit has a frequency oscillator to generate a reference frequency for down converting the received signal to the intermediate or baseband frequency for further processing. The quality of the processed signal depends on the performance of the acquisition and tracking loop circuit.
There are many problems in a mobile communication system involving cellular receivers. Examples of these problems include adjacent channel interference (ACI), Doppler effects, and multipath fading. These problems degrade the quality of the received signal by introducing noise into the signal. In addition, there may be a frequency error between the transmitted signal and the received signal. The frequency error is caused by a number of error sources. Examples of these error sources include the frequency drift in the base station, the frequency drift in the receiver unit over the operating temperature range, and handoff during changeover between base stations.
If the frequency error of the receiver relative to the transmitter is large in the presence of other channel interference or if one of the adjacent interferering channels is large and the frequency error is moderately high, a frequency tracking loop has a tendency to pull towards the interfering channel instead of towards the signal of interest (SOI). In acquisition mode, this can lead to locking to an incorrect base station. In tracking mode, this situation occurs under handoff conditions at cell boundaries when the loop filter has more noise at its input due the low carrier-to-noise ratio (CNR) and an interfering channel carrier is stronger than the signal carrier. In the short duration between handoff from the current base station to the next base station, the tracking loop may be pulled toward an interfering channel resulting in a dropped call.
A method to prevent such a false locking (i.e., the tracking loop is pulled away by an interferering channel) is to use highly stable crystal oscillator with accuracy better than +5 parts per million (ppm). However, this method requires the use of a relatively expensive crystal oscillator, resulting in higher cost for the cellular handset.
Therefore there is a need in the technology to provide a robust and economical frequency tracking system and method to reduce the effect of interference in a mobile communication system.